As explained by the boffin mastermind behind it all, the passing cars push down on a rubber mat hiding a series of tubes containing water. The movement of the fluid then runs a generator providing the Volvo with free energy. Well, the energy is free to the Volvo, but certainly not to the other cars on the road.
Even if it's clearly an improvement over, say, a hamster wheel, it doesn't exactly sound like a viable system to be laid down on every highway from sea to shining sea – but the way Volvo explained it almost makes sense. And they were nice enough to thank the passing cars for their joined effort in getting the T8 filled up to the brim. See for yourself in the video up above.